Phidgets Forums

Hi,

I made a pool ORP regulator based on arduino + phidget 1130 + ORP probe.

My ORP readings are high arount 800mV.
I am applying the owner's manual formula :
First get ADC Value (Arduino is 10bits = 1024 values), Reference Voltage is 5V :
ADC_Value = analogRead(A0) * 5 / 1023.0;
PhidgetORP = ((2.5 - ADC_Value) / 1.037)* 1000;

This formula means that positive ORPs are from 2.5V (ORP=0mV) to 0V (ORP=2000mV)
and Negative ORPs are from 2.5V (ORP=0mV) to 5V (ORP=-2000mV).
Can you please confirm this is the right formula ?

I think I will use LM4040 2.048V voltage reference in order to increase resolution and precision. That shoud allow me to measure ORP from 436mV to 2410mV.

Yes, that is the correct use of the formula. Have you tried measuring the ORP of a calibration fluid with a known ORP to see how far off the probe is? Also, if there are electrical components like pumps running in the pool it could be causing interference.

mparadis wrote:Yes, that is the correct use of the formula. Have you tried measuring the ORP of a calibration fluid with a known ORP to see how far off the probe is? Also, if there are electrical components like pumps running in the pool it could be causing interference.

Hi, thank you for fast answer.
Buying calibration fluid is my next step.
I think it will confirm I have a problem :
After 5 days without chlorine production, my ORP is still above 750mV.
I have very few changes in ORP readings : minimum is 735mV maximum is 850mV.

I bought an ORP Buffer solution (468mV).
The reading is correct, but I observe a drift in the reading.
It slowly increase... I did not wait for a stable value but it increased from 458 to 490 in around 5 minutes.
I don't know what could generate that drift.
Maybe it could be a capacitive effect ?

Try letting the probe soak in the buffer solution overnight. We've had issues with using these probes right out of the box without soaking them.

hi,

i am trying to use this code with an 1130 adapter and an ORP sensor and using this code;

delay(200);
float ADC_Value = analogRead(A0) * 5 / 1023.0;
float PhidgetORP = ((2.5 - ADC_Value) / 1.037)* 1000;

I get readings of PhidgetORP currently around -50mv for my pool water which makes no sense. The value does drift and fluctuate but I have never seen values near +500mv which is what a salt water pool should be. This is using the industrial ORP sensor.

I've ordered a seperate pen type ORP tester to check and help me get some sort of alignment of values / something to compare against.

What am I doing wrong? The sensor was dry stored for a few months but I soaked it in ph4 overnight (as recommended) and still the values make no sense.

Using an Arduino and another 1130 board and PH sensor I get normal values off the PH sensor so the issue is only with the ORP sensor. Both were dry stored and cleaned overnight in ph4 fluid and Ph works fine.

Ive tried swapping 1130's and cables with the known working one and still the ORP values are way off, like -100mv or -50mv at best, sometimes +50mv, way off +500mv of what a pool should be.

The sensor is plumbed into PVC and sealed correctly.

Is the micro-switch on the board set to ORP mode?

Yes it's on ORP. I tried swapping boards, tried swapping analogue inputs on the Arduino, tried swapping cables, it all follows the same issue, just the voltage doesn't make sense or my pool is completely off the planet and drinking water is healthier than my pool.

Currently its reading an ORP of 74mv.

So the sensor is working and reading, just the scale is way out, surely I would have thought it would be around 400mv + as an ideal pool is 650mv.

logged mv from the ORP sensor, note at some parts the pool pump is actively running so I would expect skewed() results there, but the data just doesn't make any reasonable sense, or my pool is actually a creek...

Could be a grounding issue. Can you try reading with the sensor attached to a laptop running on battery power (so there are no ground loops through the charger)? You could also try taking a reading of some pool water in a glass.

-Patrick